Introduction: Calprotectin (S100A8/A9), an antimicrobial complex released by activated neutrophils, is significantly elevated in bacterial pneumonia and correlates with COVID19 severity.

Aim: Evaluate the ability of a commercial, quantitative serum calprotectin lateral flow test (LFT) to predict clinical outcomes and identify neutrophil hyperactivation in acute lung infection.

Methods: Serum calprotectin was measured by LFT and ELISA in patients hospitalised with SARS-CoV-2 infection, non-COVID lower respiratory tract infection (LRTI) and matched controls. 52 serum inflammatory markers were measured by immunoassay. Clinical status was evaluated using the WHO 7-point scale.

Results: 274 patients were included: 157 with COVID19 (age [mean±SD]64.5±13 39.5% male), 65 with LRTI (age 69.4±13.3 49.4% male) and 52 controls (age 62.5±14.2 48.1% male). Calprotectin LFT and ELISA were strongly correlated (r=0.87 p?0.0001). Calprotectin concentrations in samples obtained within 96h of hospitalisation were increased in COVID (LFT/ELISA [µg/ml] 8.3±2.4/7.7±5.6) and LRTI patients (5.5±2.8/3.1±2.5) vs. controls (4.0±2.5/2.5±2.8), (p?0.0001). Higher LFT calprotectin measurement at baseline identified patients who failed to recover by day 29 (p=0.001 comparing those still hospitalised vs discharged by day 29), was predictive of 29-day mortality (ROC analysis AUC 0.62 95%CI 0.52-0.73), and correlated with neutrophil activation markers (proteinase3 r=0.39; azurocidin 1 r=0.32; citH3 r=0.33) and blood neutrophil counts (r=0.31)(p?0.0001).

Conclusion: Serum calprotectin LFT and ELISA were comparable and moderately associated with neutrophil activation and clinical outcomes.